African monsoons, Part 2: Synoptic-scale wave disturbances in the intertropical convergence zone over north Africa

In this part, the paper discusses several aspects of the origin, structure, development and movement of wave disturbances over the North African tropical zone during the northern summer. Analyzing the cases often actual wave disturbances which later in their life cycles developed into hurricanes over the Atlantic, it finds that though the horizontal and vertical shear of the mean zonal wind associated with the mid-tropospheric easterly jet over Africa satisfies the condition of dynamical instability under certain restrictive boundary conditions, it is the influence of a large-amplitude baroclinic wave in mid-latitude westerlies upon a stationary wave in the mountainous region of the east-central north Africa that appears to trigger the birth of a wave disturbance in the intertropical convergence zone over the Nile valley of Sudan between the Marra and the Ethiopian mountains. Physical processes likely to be important in the formation, development and movement of the disturbances are pointed out.

Millennial variability of terrigenous transport to the central-southern Peruvian margin during the last deglaciation (18–13 kyr BP)

Reconstructing precipitation and wind from the geological record could help to understand the potential changes in precipitation and wind dynamics in response to climate change in Peru. The last deglaciation offers natural experimental conditions to test precipitation and wind dynamics response to high latitude forcing. While considerable research has been done to reconstruct precipitation variability during the last deglaciation in the Atlantic sector of South America, the Pacific sector of South America has received little attention. This work aims to fill this gap by reconstructing types of terrigenous transport to the central-southern Peruvian margin (12° S and 14º S) during the last deglaciation (18–13 kyr BP). For this purpose, we used grain-size distribution in sediments of marine core M77/2-005-3 (Callao, 12º S) and G14 (Pisco, 14º S). We analyzed end-members (EM) to identify grain-size components and reconstruct potential sources and transport processes of terrigenous material across time. We identified four end-members for both Callao and Pisco sediments. In Callao, we propose that changes in EM4 (101 μm) and EM2 (58 μm) contribution mainly reflect hydrodynamic energy and diffuse sources, respectively, while EM3 (77 um) and EM1 (11 μm) variations reflect changes in aeolian and fluvial inputs, respectively. In Pisco, changes in the contribution of EM1 (10 μm) reflect changes in river inputs while EM2 (52 μm), EM3 (75 μm) and EM4 (94 μm) reflect an aeolian origin linked to surface winds. At millennial-scale, our record shows an increase of the fluvial inputs during the last part of Heinrich Stadial 1 (~ 16–14.7 kyr BP) at both locations. This increase was linked to higher precipitation in Andes related to a reduction of the Atlantic Meridional Overturning Circulation and meltwater discharge in North Atlantic. In contrast, during Bølling-Allerød (~ 14.7–13 kyr BP), there was an aeolian input increase, associated with stronger winds and lower precipitation that indicate an expansion of the South Pacific Subtropical High. These conditions would correspond to a northern displacement of the Intertropical Convergence Zone-South Subtropical High system associated with a stronger Walker circulation. Our results suggest that variations in river discharge and changes in surface wind intensity in the western margin of South America during the last deglaciation were sensitive to Atlantic Meridional Overturning Circulation variations and Walker circulation on millennial timescales. In the context of global warming, large-scale precipitation and fluvial discharge increases in the Andes related to Atlantic Meridional Overturning Circulation decline and southward displacement of the Intertropical Convergence Zone should be considered.

KETERKAITAN PERIODISITAS CURAH HUJAN DI DAERAH PESISIR DAN PEGUNUNGAN PROVINSI JAWA TIMUR DENGAN VARIABILITAS CUACA SKALA GLOBAL DAN REGIONAL

Intisari Jawa Timur merupakan wilayah yang memiliki variasi curah hujan yang dipengaruhi oleh fenomena cuaca global dan regional seperti Dipole Mode, El Nino Southern Oscillation (ENSO), Intertropical Convergence Zone, Madden Julian Oscillation, dan monsun. Topografi yang beragam juga menjadi faktor yang memengaruhi curah hujan di daerah Jawa Timur. Berbagai indeks digunakan untuk melihat aktivitas-aktivitas fenomena cuaca tersebut, seperti DMI untuk aktivitas Dipole Mode, NINO 3.4 untuk aktivitas ENSO, Indeks RMM untuk aktivitas MJO, WNPMI dan, AUSMI untuk aktivitas monsun. Pada penelitian ini digunakan analisis spektral dengan menggunakan metode Fast Fourier Transform untuk melihat periodisitas indeks masing-masing terhadap periodisitas curah hujan dari data 11 pos hujan yang terbagi menjadi 6 pos hujan daerah pesisir dan 5 pos hujan daerah pegunungan. Hasil dari penyeragaman periodisitas fenomena cuaca dengan curah hujan antara lain Dipole Mode (periodisitas 18 bulan), ENSO (periodisitas 18 dan 40 bulan), dan MJO (periodisitas 2 dan 3 bulan). Fenomena yang memengaruhi curah hujan di daerah pesisir maupun pegunungan secara dominan adalah fenomena monsun dengan diikuti ITCZ. Fenomena lain yang memengaruhi di daerah pesisir antara lain dominan MJO, serta fenomena ENSO dan Dipole Mode yang memengaruhi daerah Lamongan, Bunder, dan P3GI dengan kecenderungan lebih kuat pada fenomena Dipole Mode. Sementara itu, fenomena yang memengaruhi hujan di daerah pegunungan secara dominan adalah ENSO. Adapun fenomena lain yang memengaruhi hujan di daerah pegunungan antara lain fenomena MJO di daerah Tosari, serta daerah Kebon Teh Wonosari yang memiliki kecenderungan dipengaruhi oleh fenomena Dipole Mode meskipun pengaruhnya tidak signifikan. Abstract East Java is a region whose variations in rainfall are influenced by global and regional weather phenomena such as Dipole Mode, El Niño Southern Oscillation (ENSO), Intertropical Convergence Zone, Madden Julian Oscillation, and monsoons. Diverse topography is also a factor affecting rainfall in the area of East Java. Various indices are used to observe the activities of the weather phenomenon, such as DMI for Dipole Mode activities, NINO 3.4 for ENSO activities, RMM Index for MJO activities, as well as WNPMI and AUSMI for monsoon activities. In this study, spectral analysis was used by utilizing the Fast Fourier Transform method to see the periodicity of each index against the periodicity of rainfall from the 11 rainwater data points, which were divided into 6 coastal data points and 5 mountainous data points. Uniformity of weather phenomena with rainfall result among others Dipole Mode (18 months periodicity), ENSO (18 and 40-month periodicity), and MJO (2 and 3-month periodicity). Phenomena that affect rainfall in coastal and mountainous areas predominantly are monsoon, followed by ITCZ. Other phenomena affecting the coastal area include MJO dominant, and the ENSO and Dipole Mode phenomena that affect the Lamongan, Bunder, and P3GI regions with a stronger tendency to the Dipole Mode phenomenon. Another phenomenon that influences rain in the mountainous area is dominantly ENSO, while other phenomena include MJO phenomena in the Tosari area and Kebon Teh Wonosari region which has a tendency to be influenced by the Dipole Mode phenomenon despite the insignificant effect.

Objective Identification and Characterisation of Pacific ITCZs in ERA5 and CMIP5 models and their representation in RCMs

<p>The Intertropical Convergence Zone (ITCZ) is recognised as the most crucial feature of the tropical climate producing more than 30% of the global precipitation. Its variability dramatically affects the people living in tropical areas. In the eastern Pacific, a pair of ITCZ, one at each side of the equator, during the boreal spring is evident. It is known as the Double Intertropical Convergence Zone (DITCZ). Generally, the ITCZ in the Pacific is located in the Northern Hemisphere (NH); however, during extreme El Niño events, it can cross the equator, or a wide band of deep convection extending over both hemispheres is to be observed. The DITCZ exists more frequently and with much more strength in General Circulation Models (GCMs), resulting in a spurious bias. The DITCZ bias has been a long-standing tropical bias in climate model simulations since the early beginning. Despite the intense research on the tropical climate and its features, fewer studies investigated the state of the ITCZs through an objective and automated algorithm. Also, much fewer studies have applied such an algorithm to the GCMs output. Unfortunately, far too little attention has been paid to examining how DITCZ bias is transmitted to Regional Climate Models (RCMs). Furthermore, the input variables to the RCM from GCM are prognostic such as wind, temperature and humidity. Since precipitation is not an input, it would be interesting to examine how the representation of ITCZs in the GCMs is unfolded in the RCMs. The method adopted in this study depends on an objective and automated algorithm developed and modified by earlier studies. The algorithm uses layer- and time-averaged winds in the lower troposphere (seven layers between 1000 and 850 hPa), in addition to wet-blub potential temperature, to automatically detect the centre latitude of the ITCZs. Furthermore, it uses GPCP or CMIP5 model precipitation to obtain the extents (i.e. boundaries) of the ITCZs and the precipitation intensities. From reanalysis datasets, the NH ITCZs are found near 8°N, while the Southern Hemisphere (SH) ITCZs are near 5°S. In CMIP5 models, the DITCZ is much stronger and more frequent, and it occurs year-round. Generally, the NH ITCZs are located between 8°N and 10°N while the SH ITCZs are located between 5°S and 10°S. Moreover, models overestimate the tropical precipitation mainly, the centre and full ITCZ intensities. Furthermore, it indicates more vigorous convection in the NH ITCZs than in the SH ITCZs. The study also found that the state of ITCZ in GCMs directly influences the bias in RCM monthly precipitation. However, it depends mainly on the RCM employed. The most affected nations by DITCZ bias are Ecuador and Peru. Quantitative in-depth analysis of precipitation of GCMs and RCMs is still <span>on</span>going.</p>

An Intertropical Convergence Zone shift controlled the terrestrial material supply on the Ninetyeast Ridge

Among various climate drivers, direct evidence for the Intertropical Convergence Zone (ITCZ) control of sediment supply on the millennium scale is lacking, and the changes in ITCZ migration demonstrated in paleoclimate records need to be better investigated. Here, we use clay minerals and Sr-Nd isotopes obtained from a gravity core on the Ninetyeast Ridge to track the corresponding source variations and analyze the relationship between terrestrial material supplementation and climatic changes. On the glacial-interglacial scale, chemical weathering weakened during the North Atlantic cold climate periods, and falling sea level hindered the transport of smectite into the study area due to the exposure of islands. However, the influence of the South Asian monsoon on the sediment supply was not obvious on the millennium scale. We suggest that the north-south migration of the ITCZ controlled the rainfall in Myanmar and further directly determined the supply of clay minerals on the millennium scale because the transport of smectite was highly connected with ITCZ location. Furthermore, the regional shift of the ITCZ induced an abnormal increase in the smectite percentage during the late Last Glacial Maximum (LGM) in our records. The smectite percentage in the studied core is similar to distinct ITCZ records in different time periods, revealing that regional changes in the ITCZ were significantly obvious, and that the ITCZ is not a simple N-S displacement and closer connections occurred between the Northern-Southern Hemispheres in the eastern Indian Ocean during the late Last Glacial Maximum (LGM).

The Unexpected Relationship Between Extreme Shear Events and AMO

The vertical shear–the change in wind speed with height-of horizontal winds is a serious threat to the safety of aircraft. Yet their global distribution is not fully understood. We creatively used a precise method to calculate different types of vertical shear at four isobaric surfaces during the period of 1979~2018. The occurrence of severe shear events has increased by 19%, and they mostly occur over the equatorial ocean and within the mid-high latitude zone of the Northern hemisphere, while light shear event occurrence has been reduced by 21%. Variations of severe shear are modulated by the Atlantic Multidecadal Oscillation (AMO), which affects the frequency of shear events by influencing the intertropical convergence zone (ITCZ). Our study implies that severe shear events are regulated by internal climate variability.